An electrorheological fluid is typically formed of a suspension of hydrophilic particles in a hydrophilic liquid. When an electric field is applied across the electrorheological fluid, the particles form chains parallel to the electric field changing properties such as: the viscosity, stiffness, thermal conductivity and optical transmission of the electrorheological fluid. In past applications, the control of the electrorheological fluids has been "open-loop", in which an electric field was applied to control the viscosity, stiffness, thermal conductivity or optical transmission of the electrorheological fluid. These "open-loop" approaches to control the state of the electrorheological fluid have been hampered by the strongly time-dependent and non-linear behavior of the electrorheological fluids. Short periods between field activation yield strong action, while periods of inactivity yield slower and weaker action because the particles in the electrorheological fluid scatter with time.
The effects of temperature and humidity also contribute to a wide variation in open-loop speed and strength of response. The inventors have identified a mechanism by which closed loop feedback of one of the physical properties of the electrorheological fluid can be used to increase the response of the electrorheological to a change from a current state to a desired state and precisely control the properties of the electrorheological fluid such as the viscosity, the stiffness, the thermal conductivity or the optical transmission of the electrorheological fluid.